Caller ID says "MARKETING SOL".  They leave no message.

I call back after hours (using *67 to block my number).  Their message is "Press 9 to be put on the do-not-call list."  Then they hang up.  (I never do this as it can make things worse).

The caller ID sure makes it sounds like they are selling something.  Failing to identify themselves on their outgoing message makes me think this is a SCAM.

The short "Press 9" message is often associated with either the IRS scam or the credit card rate reduction scam.